Natural gas is increasingly vital as a global hydrocarbon resource, with 65 years of reserves based on current consumption. World reserves of natural gas are estimated at 5000 trillion cubic feet (TCF), but with 3000 TCF of this categorized as remote or stranded gas. The cost of using these stranded gas fields makes them uncompetitive. Because methanol is cheap to transport and easy to use, it has been considered as an alternative to LNG as a means of transporting gas from remote fields. However, conventional technologies are not able to produce methanol economically enough for this, so a cheap means of converting natural gas to methanol could enable these remote fields to become economically viable.

Starchem Inc has developed technology which allows 9000 tons/day of methanol to be produced economically for 20 years from a remote gas field containing at least 2.2 TCF, of which there are 200 such fields known.

Advantages of methanol

Fuel grade methanol offers many advantages compared with LNG, particularly where there is no existing LNG infrastructure. Methanol can be transported using conventional tankers, without the substantial capital cost of special tankers and regasification installations at the user’s end normally associated with an LNG system. Methanol is a clean fuel without sulphur or nitrogenous components, and can be burnt in a conventional combined cycle plant without emission reduction systems.

Process features

The Starchem methanol production process uses commercially proven processes arranged to deliver large capacity methanol production at substantially reduced capital cost. Well proven, high air extraction gas turbines from GE (see table) form a key part of the system.

The process integrates enriched air production, catalytic partial oxidation, methanol synthesis and purge gas hydrogen recovery. The steps in the process flow scheme are:

  • Air drawn into suction of a gas turbine.
  • Much of the air is extracted from the gas turbine compressor through a booster compressor and into air enrichment membranes. The rest goes to the combustor or is used for internal component cooling.
  • Enriched air is produced by the membranes. These work because oxygen diffuses faster across the membrane than nitrogen. The major part of of the nitrogen and some residual oxygen remains in the main gas stream as depleted air.
  • Depleted air is heated against compressor discharge air, and returns to the gas turbine as secondary air to the combustor, forming a significant part of the mass flow through the turbine.
  • Enriched air is compressed, heated and fed to catalytic partial oxidation section.
  • Natural gas is heated, desulphurized, mixed with steam, heated further and fed to the catalytic partial oxidation reactor.
  • Synthesis gas is cooled, recovering useful heat, and is then compressed to a suitable pressure for methanol synthesis.
  • Methanol is synthesized in a cascade of reactors. The crude methanol passes to a methanol distillation section where it is stabilized and reduced to an economic water content for transport.
  • Purge gas from the methanol synthesis cascade is treated to recover hydrogen to recycle with the tail gas passing to the gas turbine as fuel.

    Economics of the process

    The process has a very low capital intensity compared to alternatives because:

  • The process employs, as a fundamental part of the plant, a GE gas turbine with a double duty: low cost source of power to operate the process; and low cost source of air for enrichment. Selection of the GE turbine is fundamental to the attractive economics and reliability of the process.
  • The process uses membranes rather than cryogenics to produce enriched air. This has the benefit of low cost, and an operability advantage as enriched air becomes available virtually instantaneously, in contrast to a cold box, requiring many hours.
  • Conventional methanol processes use a synthesis loop in which recycled synthesis gas and built up inerts are mixed with fresh gas prior to feeding to the reactor. Thus the reactor sees a highly diluted and not very reactive gas. In contract, the Starchem process has synthesis gas passing through a cascade of reactors. This provides several advantages: the first reactors in the cascade see reactive gas, with only the final reactor having conditions comparable to a reactor in a synthesis loop; managing composition and temperature profiles can minimize life cycle costs; and a combination of isothermal and adiabatic reactors can minimize costs.
  • Conventional methanol processes produce a hydrogen rich purge gas stream which, if no hydrogen consumer is available, can only be used as furnace fuel. The Starchem process starts with substoichiometric syngas, and can beneficially recycle all available hydrogen, leaving just enough residual tail gas to fuel the gas turbine, which is an integral part of the process. Both invested capital and operating costs are lower than conventional methanol production. Capital cost reduction is estimated at 25-40 per cent, with production costs reduced by up to $50/tonne from a typical cost of $100-110/tonne.

    GE ‘E’ system gas turbines